Role of sieve-element occlusion proteins in root pathogenesis and phloem signalling

• Applicant: Dr. Alexandra Furch
• Subject Area: Plant Physiology ; Plant Breeding and Plant Pathology
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 266247023

The phloem is the long-distance pathway for photoassimilates and several other components, and it is a preferred pathway for pathogen movement. Compromised phloem functionality may have disastrous consequences for plant health and development. The project will investigate the molecular and cellular basis of modifications to long-distance phloem transport induced by different biotic stresses. The starting point of this project elucidates the role of Sieve Element Occlusion (SEO) proteins in Arabidopsis. Since the SEO gene family was identified very recently, a potentially more general role for these proteins in plant health and development is yet unexplored. Using single knock-out (KO) mutants, it will be investigated if and how two members of the SEO protein family (AtSEO-a and AtSEO-b) contribute to phloem-based defence signalling and the immunity status of the plant. Preliminary work has shown that biotic triggers, such as the bacterial microbe associated molecular pattern (MAMP) flagellin, elicit sieve element occlusion in remote plant parts. Thus, the role of SEOs in MAMP-induced root-to-shoot signalling will be investigated using seo-a and seo-b mutants, as well as AtSEO-a/b overexpressors. MAMP triggered responses will be analyzed in regard to: i) electropotential waves (EPWs), ii) Ca2+ spiking, iii) phytohormones, and iv) the interplay between these signals and structural modifications, focusing on the occlusion mechanisms of sieve elements. The pathogen-triggered EPWs will be extra- and intracellularly studied with microelectrodes, locally and systemically in different vascular cells. MAMP-induced changes in Ca2+ levels will be detected using Arabidopsis seedlings expressing the fluorescence resonance energy transfer (FRET)-compatible Ca2+ sensor YC3.6. The seo mutants and AtSEO overexpressors will be monitored for phytohormone (JA, cis-OPDA, JA-Ile, SA, ABA) dispersion in roots, shoots and leaves to diverse microbes, including viral, bacterial, and fungal pathogens. In order to elucidate the relationship between MAMP-triggered sieve element occlusion and immunity status, the seo mutants and AtSEO overexpressors will be monitored for altered resistance to diverse microbes, including viral (Turnip Crincle Virus), bacterial (Pseudomonas syringae pv tomato DC3000), and fungal (Golovinomyces orontii) pathogens. Arabidopsis lines expressing AtSEO proteins tagged with the fluorescent reporter protein (hrGFP) will be used to characterize the location of SEOs in MAMP-treated and non-treated plants. The promoter activity and tissue specificity of the identified Arabidopsis SEOs will be investigated. MAMP-triggered growth alteration in shoots and roots will be investigated; this will be correlated to the nutrient distribution in the phloem system and analyzed in SEO mutants by use of carboxyfluorescein.

• DFG Programme: Research Grants